Ah, yes, that'll be the reason...ha!

Why men like to marry younger women
The reason that men like to marry women who are years younger than themselves has been discovered: they have more grandchildren as a result.

A study published today in the journal Biology Letters, provides evidence that the reason for these unions is that men prefer young women due to their high fertility while women prefer older men due to their wealth and high social status, which make them good providers for the offspring.